I. Field of the Invention
The present invention is directed to a navigable downhole drilling system for directional drilling and, in particular, to a "smart" driller which continuously monitors and corrects the path of the directional drilling for optimum positioning of the borehole.
II. Description of the Prior Art
Directional drilling has become increasingly important in the exploration for fossil fuels as well as the extraction of environmentally hazardous materials from the earth. Directional drilling facilitates penetration scattered fuel deposits from a single surface well or horizontal penetration to improve extraction. However, as the depth increases and precision of directional drilling becomes increasingly important, an accurate determination of the positioning of the drill bit or the downhole drilling system is necessary. Early downhole drilling systems relied upon calculations as to position based upon the total length of drilling string and the kick-off or build rate of the drilling system. However, such directional drilling can be affected by unknown factors such as the formations through which the drilling system must pass. Although a reasonable determination of position could be calculated, precise positioning was unknown.
Measurement-While-Drilling or MWD's have become widely accepted as a means of monitoring the direction and position of the drilling system. MWD's transmit a signal pulse to the surface which provides information relating to total depth and inclination. However, it can take several seconds for the information to reach the surface and several additional seconds before a course correction can be instituted at which time the information may no longer be accurate. In addition, transmission of the data is subject to several types of interference. MWD's typically utilize strain gauges to determine bending of the external casing which may be a result of the proper build rate or an encounter with an unanticipated formation. Finally, MWD's are added to the drilling system increasing the overall length of the drilling system. As length of the drilling system is increased potential build rate is sacrificed.
The prior known drilling systems do not incorporate means for monitoring and adjusting the direction of drilling. Although the direction of drilling can be controlled from the surface by varying thrust drill pipe orientation, and drilling fluid, an optimum system would carry out course corrections as new formations are encountered, etc. Such a downhole system would eliminate the delay associated with the transmission of information to the surface and subsequent correction. Consequently, only intermittent transmission of data would be necessary to keep the surface rig informed of drilling progress. Alternatively, a signal could be transmitted only when it becomes necessary to vary parameters controlled at the surface. The prior art systems are not capable of such sophisticated directional drilling.